When a sample to be measured is irradiated with terahertz radiation, a measurement result according to the physical properties of the sample can be obtained. For example, when the sample is irradiated with terahertz radiation having different wavelengths, terahertz waves having specific wavelengths are absorbed in the sample by electromagnetic radiation absorption of the sample. Therefore, the terahertz radiation transmittance versus frequency characteristic emerges in a specific waveform according to the physical properties of the sample.
A known method for measuring the terahertz radiation transmittance is terahertz time-domain spectroscopy (hereinafter, referred to as THz-TDS). Example 2 in Patent Literature 1 discloses a THz-TDS method using a reflecting optical system. In this example, the terahertz radiation emitted from a terahertz radiation generator is focused through parabolic mirrors onto the sample, and the terahertz radiation reflected on the sample is then focused to a terahertz radiation detector using parabolic mirrors.
Patent Literature 1: Published Japanese Patent Application No. 2008-83020